/Engine/Source/Runtime/InteractiveToolsFramework/Public/InteractiveTool.h
Indexof the watcher, which can be used in SilentUpdateWatcherAtIndex
template<typename PropType> int32 WatchProperty ( typename [TPropertyWatcher](API\Runtime\InteractiveToolsFramework\FWatchablePropertySet\TPropertyWatcher)< PropType >::FValueGetter GetValueIn, typename [TPropertyWatcher](API\Runtime\InteractiveToolsFramework\FWatchablePropertySet\TPropertyWatcher)< PropType >::FChangedCallback OnChangedIn, typename [TPropertyWatcher](API\Runtime\InteractiveToolsFramework\FWatchablePropertySet\TPropertyWatcher)< PropType >::FNotEqualTestFunction NotEqualsIn )